I. Field of the Invention
The present invention relates to an active suspension apparatus using hydropneumatic springs for suspending a vehicle body, i.e., the sprung mass on wheels, by utilizing fluid and gas pressures.
II. Statement of the Prior Art
Vehicles travel on smooth and rough roads in a wide velocity range between a low speed and a high speed. Under these circumstances, all possible travel conditions must be assumed and optimal control for rolling and pitching must be performed to satisfy a given travel condition. More specifically, when the vehicle travels along a smooth paved road, a relatively soft suspension is required. However, when a vehicle with a soft suspension rides over a joint in a pavement, the damping force is too small for the comfort of passengers. When the vehicle travels along a rough road, a relatively hard suspension is required. However, vibrations which resonate with internal organs cause discomfort to passengers. Thus, the damping force of the suspension system must be optimally controlled.
In a conventional vehicle suspension/damping apparatus having variable damping force characteristics, a vehicle velocity, a relative displacement between the body and the wheels, a relative velocity, and accelerations in the passenger compartment and the axle must be evaluated to change the damping forces. This conventional control technique is not based on the riding comfort of the passengers but on simple level control or control for a predetermined period of time. In this manner, the control operation does not precisely correspond to circumstances of the passenger's compartment in which the passenger ride.
In the conventional apparatus, an acceleration detecting means is arranged at part of the vehicle body to discriminate motion and vibrations of the body, thereby detecting the back-and-forth acceleration and deceleration and the vertical acceleration. However, the acceleration detecting means which is mounted at a position excluding a passenger's position is subjected to vibrations of the vehicle frame and constituting components such as auxiliary parts. As a result, vibrations felt by passengers cannot be precisely evaluated, resulting in inconvenience.
The riding comfort of passengers during travel is evaluated as soft or hard and stiff with respect to sprung resonant components. Riding sensation varies in accordance with passengers. When a driver personally controls riding comfort, a state evaluation standard which is closely associated with the passenger's comfort must be established to control the damping force of the vehicle.